AD,
Well, it depends upon what you want. You can create reservations using MD21, which can represent future receipts of goods. You can also create reservations that represent exploded component demand. These supply and demand objects can be configured so as to allow SAP MRP to consider them.
The devil, however, is in the details. With Reservations, there is no automatic explosion of demand for component parts. Also, during this interim period, you have to figure out how to report goods issues against the components of your production orders, goods receipts against your production orders; and also how to report the completion or cancellation of your production orders to SAP.
Inevitably you will end up with a large manual effort to maintain these reservations, or you will have to embark on a programming effort, to ensure that your production order system stays consistent with your reservations. Do you really want to do this, when your 'next step' is to replace this solution when you fully integrate your production system?
Best Regards,
DB49